Allow union type fields to be match fields

In LOOKUP JOIN, union type fields don't work as match fields because we
extract their names wrong. Correct this and make MatchConfig rely on
FieldName to avoid this problem going forward.

Author: alex-spies

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/enrich/LookupFromIndexOperator.java

@@ -23,6 +23,7 @@
 import org.elasticsearch.core.Releasables;
 import org.elasticsearch.tasks.CancellableTask;
 import org.elasticsearch.xcontent.XContentBuilder;
+import org.elasticsearch.xpack.esql.core.expression.FieldAttribute;
 import org.elasticsearch.xpack.esql.core.expression.NamedExpression;
 import org.elasticsearch.xpack.esql.core.tree.Source;
 import org.elasticsearch.xpack.esql.core.type.DataType;
@@ -45,7 +46,7 @@ public record Factory(
         DataType inputDataType,
         String lookupIndexPattern,
         String lookupIndex,
-        String matchField,
+        FieldAttribute.FieldName matchField,
         List<NamedExpression> loadFields,
         Source source
     ) implements OperatorFactory {
@@ -56,7 +57,7 @@ public String describe() {
                 + " input_type="
                 + inputDataType
                 + " match_field="
-                + matchField
+                + matchField.string()
                 + " load_fields="
                 + loadFields
                 + " inputChannel="
@@ -76,7 +77,7 @@ public Operator get(DriverContext driverContext) {
                 inputDataType,
                 lookupIndexPattern,
                 lookupIndex,
-                matchField,
+                matchField.string(),
                 loadFields,
                 source
             );

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/planner/LocalExecutionPlanner.java

@@ -766,10 +766,10 @@ private PhysicalOperation planLookupJoin(LookupJoinExec join, LocalExecutionPlan
         );
     }
 
-    private record MatchConfig(String fieldName, int channel, DataType type) {
+    private record MatchConfig(FieldAttribute.FieldName fieldName, int channel, DataType type) {
         private MatchConfig(FieldAttribute match, Layout.ChannelAndType input) {
             // Note, this handles TEXT fields with KEYWORD subfields
-            this(match.exactAttribute().name(), input.channel(), input.type());
+            this(match.exactAttribute().fieldName(), input.channel(), input.type());
         }
     }